I'm in the process of restoring an sx-71. When I got it, it had virtually no reception above the broadcast band and the S meter did not function.I did the usual, replaced the lytic and paper caps, out of spec resistors, cleaned all switches and pots, checked tubes, etc.Went through the alignment also.The radio now has sensitivity and the S meter works on all bands, except band 3.On band 3 the meter stays all the way to the right like when the radio is off and I can only hear my signal generator with no band noise.I'm wondering if its something in the bandswitching circuitry.

Hang your DC voltmeter on the AGC line and measure the voltage with a moderately strong station tuned in. Tune to a quiet spot on the same band and note the AGC voltage with that condition. This will tell you in which direction the voltage swings for strong signals and weak signals.

Switch to the band that's dead and do the same. Betcha' the AGC thinks Radio Habana is next door and it needs to go deaf.

Figure out why.

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Chuck McGregor re-drew the SX-71 IF section schematic with notations.He commented on Antique Radio restoration forum: "Unlike the SX-42 and SX-62 which run the First IF at fixed gain using a 270 ohm cathode resistor to ground, the SX-71 returns both the First (V4) and Second IF (V5) cathodes to the wiper of the sensitivity control, through different resistances and changes those resistances for the Second IF on a band-by-band basis (Switch 1, Section 5 [rear] wafer).It would be great to find an explanation of WHY, from the guy who designed it ..."http://pcbunn.cacr.caltech.edu/jjb/Hallicrafters/SX71/SX71-2ndIFa-Chuck.gif

IF you only have issue on Band 3, LOOK carefully (magnifying lens) forcontact corrosion, carbon blackening, or broken contact fingers on the band switch wafer.SWITCH 1, SECTION 5, REAR WAFER.See the higher Second IF (V5) cathode resistance for Band 3, switching in R32 (2700 ohms)??CAREFULLY check this area of circuit and measure R32.http://pcbunn.cacr.caltech.edu/jjb/Hallicrafters/SX71/SX71-2ndIFa-Chuck.gif

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